Process and reagent for carbon monoxide detection



April 12, 1966 Filed April 29, 1965 E. s. MAYO. JR 3,245,917

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April 12, 1966 S MAYO, JR i I v PROCESS AND REAGENT FOR CARBQN monoximans'rnc'r on' I Filed April 29, 1965 2 sheetsheet 2 3,245,917 PROCESS AND REAGENT FOR CARBON MONOXIDE DETECTION Edmund S. Mayo, Jr., Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Apr. 29, 1963, Ser. No. 276,246 2 Claims. (Cl. 252-408) This invention relates to the detection of noxious gases by chemical action and particularly to an improved chemical reagent for detecting the presence of carbon monoxide and. to apparatus for advantageously employing this reagent.

Chemicals such as those employed in Shepherd tubes or in 1 -80; and palladium chloride systems have been used to indicate the presence of carbon monoxide by undergoing color changes when exposed thereto. These chemicals, however, leave much to be desired in the way 9f possessing the combined characteristics of high sensitivity to low concentrations of gas and attention-free stability over long periods of time. Palladium chloride (PdCl in particular, has the requisite sensivity, but in the solution form or embodied with other indicating reagents, is prone to chemical deterioration within a period an improved palladium chloride-containing reagent for detecting such gas and which is self-regenerative to extend the usefulness and life of the reagent for as much as six months and longer.

A still further object of the present invention is to provide a carbon monoxide detector which is self-regenerative and which is in dry particulate form for easy handling. Another object of the present invention is to provide a chemical reagent which rapidly indicates the presence of sub-toxic concentrations of carbon monoxide and which is sclfregenerative to the extent so as not to be adversely affected by long term exposure thereto.

Still another object of the present invention is to provide a process for the preparation of the improved chemical reagent.

A still further object of the present invention is to provide an economical apparatus for utilizing the improved chemical reagent for the continuous monitoring of CO well below the toxic limit.

And still a further object of the present invention is to provide an apparatus having the foregoing characteristics and in addition a unique construction for promoting sampling of the surrounding atmosphere and for photoelectrically sensing color changes of the chemical reagent. Other objects will appear hereinafter.

These and other'objects are accomplished in part by the reagent consisting essentially of palladium chloride and a regenerating amount of hydrochloric acid with re spect to the palladium chloride, both adsorbed onto an inert porous matrix or solid carrier such as silica gel. A practical regenerating amount of the hydrochloric acid is when the weight ratio thereof to PdCl is from about 0.65 to about 2.0. Other objects of the present invention are achieved by apparatus including a compact detection unit containing the improved reagent as will be later described.

To better understand the attainment of these objects "ice reference is to be made to the accompanying drawings in which: FIG. I is a graph to compare the sensitivity of the reagent before and after extended use, and in which the abscissa is the response time of the reagent at dilierent concentrations of CO plotted along the ordinate axis; and FIG. 2 depicts, in perspective, a partially cut-away view of a detector unit and, in schematic, a photoelectric alarm circuit therefor.

The carbon monoxide detection or indication by the above described reagent representatively occurs as follows:

The palladium chloride (PdCl which is light in color, is reduced by the carbon monoxide (CO) present in the surrounding atmosphere and coming into contact therewith to dark palladium metal (Pd) which is evidenced by a darkening of the reagent. The silica gel merely serves as a support for the PdCl and the HCl, and the darkening that takes place is due to the formation of metallic palladium. Inert inorganic carriers such as silica or alumina gel are preferred. The degree of darkening occurring depends upon the amount of Pd formed and thus upon the amount of CO present in the atmosphere contacting the reagent.

Dangerous concentrations of, CO could be indicated solely by this darkening etlect; however, it is preferred that an alarm or annunciator apparatus be coupled with the reagent so as to give notice of the presence of CO rather than depend upon intermittent visual observation of the reagent and judgment of the relationship of the degree of color change to gas concentration. An exemplary apparatus for this purpose will be described'hereinafter;

An important aspect of the present reagent of the :present invention is the fact that it is self-regenerative, i.e., the a above reaction that proceeds toitherightwhen CO is present is driven to the left during the absenceof significant amounts of the toxic gas. This driving force is internally supplied by the HCl originally adsorbed into silica gel along with the PdCl causes the conversion .of the palladium metal to the chloride thereof and thereby returns the gel to its original light color. The reagent is dried before use, but is preferused in the presence of a relative humidity of at least Control of the regeneration rate is important in regulat- =ing the sensitivity of the PdCl component of'the reagent.

Since indication of the presence of carbon monoxide occurs by permitting the reaction represented by the above equation to proceed to the right whereby metallic pa'lla-. diumais formed and the gel darkens, it is important that the regeneration rate be so regulated as to permitthis reaction to proceed rapidly when a sub-toxic but dangerous concentration of CO is present, but the reversal of this reaction must be allowed to proceed at such a rate as to prevent darkening by the cumulative effects of continuous exposure of the reagent to lower concentrations of the gas and other reducing materials present in the air.

The regeneration time of the reagent can be controlled by the relative amount of 'HCl with respect to Pd'Cl present, in the gel. The incorporation of too small an amount of hydrochloric acid in the gel increases the sensitivity of the PdCl and the regeneration time, but lowers the life of the reagent. The reason for this :is that'there is insufficient HCl available to attack the metallic palladium formed in the carbon monoxide detection reaction. The incorporation of too great an amount of HCl decreases the sensitivity of the reagent and increases the regeneration speed thereof and thus the overall lifetime of the reagent. The practical result of having :a proper amount of hydrochloric acid present in the origi- The presence of this acid na! gel is that the reagent is rendered capable of extended life six months or more. The preferred ratio of HCl to PdCl originally adsorbed into the silica gel is from about 0.65 to about 2.0 parts by weight of HCl per part by weight of PdCl The reagent having these proportions of chemical agents will darken when exposed to'nearly harmful (to human beings) concentrations of CO and will return essentially to its original light color when the CO concentration in the atmosphere being sampled falls below the nearly harmful levels.

In -FIG. 1.is shown a graphical comparison of the sensitivity of a particular chemical reagent of the present invention after initial and after extended exposure (contact) to CO. The response time plotted along the abscissa is the time for the reagent to reach a selected shade or degree of darkening which is the same for all concentrations. Hence, for a CO concentration of 0.04%, a fresh reagent requires about 1 minute to reach this selected color whereas the long-exposed reagent requires through and reaching ,the photodetector.

. 4 i I reagent 18 affects the amount of light transmitted there- When a certain amount of color change has occurred, corresponding to warning of the presence of these lethal concentrations of A representative reagent is prepared in the following way: To 80 cc. of a 0.5% by weight palladium chloride aqueous solution is added 0.75 cc. of concentrated hydrochloric acid (35%). To this solution is added sufiicient silica gel (Davidson Chemical Company, mesh size 28- -200) until the solution is completely absorbed. The reagent is activated by drying, which normally requires approximately 2 hours at 80 C., after which the reagent is allowed to cool. Such a gel reagent will have a regeneration time of about 10 to 20 minutes after the reagent is exposed to 0.05% CO for one minute at a relative humidity of -35% A concentration of 0.01% carbon monoxide causes sufiicient darkening of the reagent in one minute so that a proper alarm coupled therewith will be actuated.

It ispreferred although not absolutely necessary that the previously described reagent be coupled with a device for automatically detecting the darkening of the reagent. An automatic detection system which advantageously utilizes the improved reagent will now be described.

FIG. 2 shows a detector apparatus 1 consisting generally of a detection unit 2 which is placed at the location where gas monitoring is to occur and an electric circuit 4 to power unit 2, said circuit being capable of proximate or distant location with respect to unit 2. Unit 2 is composed of a housing 6 having electrical connectors such as prongs 8 and 10 protruding from the rear thereof and adapted to be plugged into a standard 115 v., 60 c. electric receptacle '(not shown). Housing 6 is compact and typically measures 1% inch in its greatest diameter and 1 inch in depth.

Within housing 6 is mounted a neon lamp 12, such as watt size, inseries with current-limiting resistor 14. Spaced from and facing lamp 12 is mounted photodetector 16 so that changes in illumination from lamp 12 are observed by the photodetector. Terminals 17 are provided for connecting the photodetector to external circuitry. The reagent 18 of the present invention is held within container 20 and together they are designated a module of reagent 22. I

In the embodiment shown, container 20 is a fine enough noncorrosive. wire screen to retain the reagent granules and is formed as an enclosure mounted on the face of photodetector 16 and filled with reagent so as to he a substantial covering thereof. Therefore, light from lamp 12 must be transmitted through reagent 18 to reach photodetector'16. Accordingly, a change in color of the The reagent is then ready for use.

the presence of a certain amount of a particular gas, the photodetector 16 is pre-set or calibrated to detect this change and to signal alarm equipment which, in turn, warns personnel of this fact. Screen20 can form one side of the container for the reagent 18 and the face of photodetector 16 the other, or screen 20 can form both sides of the container as well as theedges thereof. In the embodiment shown, a noncorrosive spacer 23 is positioned between the screen 20 and the face of the photodetector to form the container for reagent 18. Spacer 23 has a separation at the top thereof to permit loading and unloading the reagent with respect to the container.

Housing 6 is provided with a plurality of openings 24 spaced thereabout to grant access to the surrounding atmosphere which may contain the gas to be detected. Housing 6 is compact so that the heat generated therewithin by lamp 12 causes the atmosphere being sampled by reagent 18 to rise and .be replaced by cooler air from outside'the housing. The convection current thereby created insures a proper and continuous sampling to be obtained by the detection unit 2. Openings 24 in circular portion 26 of housing 6 can be 5 in number and inch in diameter and'in portion 28 can be 4 in number and A3 inch in diameter. A relative humidity of greater than 20% and preferably between 25-50% is maintained in the interior of housing 6 by passing through one of the openings in portion 28 a wick of sponge-like material (not shown), one end of which projects into the interior of portion 28' an amount equal to the radius of this portion and the other end of which is passed to a reservoir of distilled water 'via a tubular neck adjoining portion 28.

The photoelectric elements of the unit 2 are powered by circuit 4 from a power source such as 115 v., c. supplied via leads 30 and 32. These leads are connected to an isolationtransformer 34 used optionally for safety purposes. 'The lamp-12 and current-limiting resistor 14 combination is connected in series with variable resistmice 36 to provide a light intensity adjustment circuit which forms one parallel branch of the load on the secondary winding of isolation transformer 34. The second parallel branch consists of photodetector 16 connected in series with the power supply and relay'coil 38. Relay arm-40 is in the position shown in FIG. 2.

when photodetector 16 vis receiving full light from lamp 12. As the current in coil 38 decreases, however, with increased color change, e.g., darkening of PdCl upon contact with a particular gas, CO, relay arm 40 falls back onto contact 42 at a predetermined current level and time for a given gas concentration, thereby completing the circuit which actuates an alarm such as horn 44. Variable resistance 36 serves as a zero adjustment, being set initially to allow sutlicient current to pass through coil'38 just below a certain level of gas concentration,

usually below the toxic limit, to just hold relay arm 40 in the position shown in the drawing.

To operate the gas detection apparatus of the present invention, the detection unit Zneed only be plugged into a receptacle which is coupled with a circuit such as circuit 4 and thereafter the surrounding atmosphere is con- .tinuously convected through and sampled by the unit 2 tacts the reagent therein to be sampled for the presence of CO. The module of reagent 22 can also be vertically standing and instead of having a screen container, the container can be of solid transparent, e.g., an open top wide mouth glass vial, or of a non-transparent material,

such as noncorrosive metal, having a window so that the photodetector receives light from the neon lamp after transmissive incidence through the reagent and the window of the container. In such an arrangement, light is transmitted through the solid, granular reagent by a process of multiple scattering in which the incident light is reflected from individual particles of the reagent many times. The thickness of the reagent bed in the direction parallel to .the incident beam of light from the lamp is usually chosen so that approximately 50% of the incident light passes, by scattering, through the reagent to the detector. Since the emergent light has also been multiply reflected from the surface of the reagent bed,

its color (spectral'distribution) and the amount of emer-.

gent light will be closely related to the color of the reagent.

Any photodetector 16 having the desired expanse of light sensitive area and a suitable response time may be used in the detection unit 2 hereinbefore described. An example of a suitable photodetector is the CBS-9 type available from Pioneer Electric and Research Corporation, Forest Park, Illinois.

Because of the compactness of the detector unit 2,

' ployed. For example, the detector unit may be battery powered and/or mounted onto amovable element for.

probe-type analysis.

Because of the relatively long K. w. VERNON, M. wEmBLAr'r, gaiamtzamam;

Since many widely difierent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is'not limited to the specificembodiments thereof except as defined in the appended claims.

What is claimed is:

l. A self-regenerating reagent for detecting the presence of carbon'monoxide at a relative humidity of at least 20%, consisting essentially of a carrier of silica gel, said carrier having absorbed thereon palladium chloride and a regenerating amount of hydrochloric acid.

2. A process for preparing a self-regenerating reagent for detecting the presence of carbon monoxide in an atmosphere having a relative humidityof at least 20%, which process comprises absorbing palladium chloride and hydrochloric acid from' solutions thereof onto granules of silica gel, said hydrochloric acid being present in an amount from about 0.65 to about 2.0 parts by "weight per part by weight of palladium chloride, and thereafter drying said gel at temperatures between about C. to about C.

References Cited by the Examiner UNITED STATES PATENTS 1,684,587 9/1928 Hultman 252-408 1,891,429 12/1932 Ljunggren ..-..-.23-232 2,738,257 3/1956 Darby 252-408 FOREIGN PATENTS 1 673,419 6/ 1952 Great Britain.

188,921 v1/1937 Switzerland.

OTHER REFERENCES Mellon A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. XV, page 662, pub. by Longmans, Green and Co., New York, 193 6.

McPherson & Henderson: A Course in'General Chemistry, p. 202, pub. by Ginn & C0,, New York, 19 15.

JULIUS TGREENWALD, Primary'Eimrniner. f 

1. A SELF-REGENERATING REGENT FOR DETECTING THE PRESENCE OF CARBON MONOXIDE AT A RELATIVE HUMIDITY OF AT LEAST 20%, CONSISTING ESSENTIALLY OF A CARRIER OF SILICA GEL, SAID CARRIER HAVING ABSORBED THEREON PALLADIUM CHLORIDE AND A REGENERATING AMOUNT OF HYDROCHLORIC ACID.
 2. A PROCESS FOR PREPARING A SELF-REGENERATING REAGENT FOR DETECTING THE PRESENCE OF CARBON MONOXIDE IN AN ATMOSPHERE HAVING A RELATIVE HUMIDITY OF AT LEAST 20%,, WHICH PROCESS COMPRISES ABSORBING PALLADIUM CHLORIDE AND HYDROCHLORIC ACID FROM SOLUTIONS THEREOF ONTO GRAINULES OF SILICA GEL, SAID HYDROCHLORIC ACID BEING PRESENT IN AN AMOUNT FROM ABOUT 0.65 TO ABOUT 2.0 PARTS BY WEIGHT PER PART BY WEIGHT OF PALLADIUM CHLORIDE, AND THEREAFTER DRYING SAID GEL AT TEMPERATURES BETWEEN ABOUT 80*C. TO ABOUT 90*C. 